1. Field of the Invention
The present invention generally relates to optical communication systems and, more particularly, to reducing phase jitter in an optical communication system.
2. Description of the Related Art
In the propagation of optical pulses in ultra-long haul (ULH) applications, such as transoceanic transmission, numerous parameters associated with such transmission limit the capacity of the optical fiber cable system through which the optical pulses are transmitted. Pulse timing jitter is a major limitation to the maximum reach of ULH on-off transmission systems. For single-channel systems, a significant source of timing jitter is attributed to contributions from transmitter and receiver electronics, acoustic interaction effects, or especially, in the case of soliton transmission, to the Gordon-Haus effect. The Gordon-Haus effect is caused by the interaction of soliton pulses with amplifier spontaneous emission (ASE) noise present along the transmission medium. For multi-channel wavelength division multiplexed (WDM) systems, timing jitter is increased significantly by inter-channel soliton collisions. This collision-induced (CI) timing jitter is the dominant impairment of WDM soliton systems. Thus the search continues for other transmission formats that are less susceptible to timing jitter.
One such transmission format that has attracted attention is a differential-phase-shift-keyed (DPSK) modulation format. In a DPSK system (as well as other phase-shift keying (PSK) modulation formats, such as quadrature phase-shift keying (QPSK)), information is encoded using phase differences between neighboring optical pulses (e.g., solitons). For PSK systems, however, the error-free transmission distance is limited by phase jitter, in which power shifts caused by ASE are converted into phase shifts by self-phase modulation (SPM). It is thus desirable to provide a practical and cost effective method and apparatus for reducing phase jitter in an optical communication system.